The present invention relates to prosthetic systems for the replacement of joints or portions thereof. More particularly, the invention concerns a customized prosthesis, and a method of designing and manufacturing such a prosthesis based upon computed tomography data obtained from the patient.
For treatment of various problems with the shoulder and hip such as degenerative arthritis and trauma, one method of providing relief to a patient is to replace the articulating surfaces with an artificial or prosthetic joint. In the case of a shoulder, the humerus and glenoid articulating surfaces are replaced. In the case of a hip, the femur and acetabulum articulating surfaces can be replaced.
In such replacement, pain relief, increased motion and anatomic reconstruction of the shoulder or hip joint are goals of the orthopaedic surgeon. With multiple variations in human anatomy, prosthetic systems need to accurately replicate the joints that they replace and maintain the natural symmetry of the joints on the left and right sides of the patient's body.
A hip replacement procedure may involve a total hip replacement or a partial hip replacement. In a total hip replacement procedure, a femoral component having a head portion is utilized to replace the natural head portion of the thighbone or femur. The femoral component typically has an elongated intramedullary stem which is utilized to secure the femoral component to the patient's femur. In such a total hip replacement procedure, the natural bearing surface of the acetabulum is resurfaced or otherwise replaced with a cup-shaped acetabular component that provides a bearing surface for the head portion of the femoral component.
Acetabular cups may be secured to the acetabulum in a number of different ways. For example, an acetabular cup may be secured to the acetabulum by the use of bone cement. However, recent studies have speculated that it may be desirable to secure artificial components to natural bone structures without the use of bone cement. Hence, a number of press fit acetabular cups have been designed for cementeless securement.
In either case (i.e. cemented or cementless), the acetabulum is first reamed by the surgeon in order to create a cavity into which the acetabular cup is secured by the use of a surgical tool known as a reamer. It is often difficult for the surgeon to properly match the size of the reamer to the desired acetabular cup size.
Although press fit acetabular cups have heretofore been referred to as being “generally hemispherical” in shape, such cups, in reality, are sub-hemispherical in shape. Such a configuration has a number of drawbacks associated therewith. For example, if the acetabular cup is not truly hemispherical, it may be difficult for the surgeon to ream a properly sized cavity in the acetabulum. In particular, the cutting heads of reamers are typically configured as relatively true hemispheres. Hence, when a surgeon reams the patient's acetabulum, the surgeon has to “estimate” the approximate depth of the reamed recess. More specifically, if the surgeon reams too far, the annular rim of the acetabular cup will be recessed in the reamed cavity. Conversely, if the surgeon does not ream deeply enough (i.e. “under reams”), the acetabular cup will not be fully seated in the reamed cavity of the acetabulum. In light of the fact that surgeons occasionally select a reamer that is slightly smaller in size than the acetabular cup to be implanted, under reaming may also disadvantageously lead to bone fracture of the acetabulum since excessive force is often utilized to insert the cup into the undersized (i.e. under reamed) cavity. Some of the early bone cemented cups did not suffer from this problem because they were configured more closely as “true” hemispheres. However, as indicated above, such cups undesirably required the use of bone cement during implantation thereof.
Another drawback associated with press fit acetabular cups relates to the configuration of the outer shell. In particular, in an attempt to increase retaining forces, a number of acetabular cups have been designed with a flared rim (known as dual radius or “bubble” cups) or a frusto-conically shaped annular rim portion (known as dual-geometry cups). Although the configuration of such cups may generate relatively strong retention forces at the rim portion of the cup, surface contact and therefore retention forces are relatively small at the portions of the outer shell other than the rim portion, particularly in the dome area. Moreover, such reduced surface contact at the portions of the outer shell other than the rim portion reduces bone ingrowth in such portions.
With the above-mentioned press-fit acetabular cups, a two-part reaming process is typically necessary. The two-part reaming process involves reaming of the acetabulum using a reamer of a first size, then reaming the acetabulum using a reamer of a second size. The more reaming, the more likely that a problem will occur. For example, many conventional cementless acetabular cup systems use a cup that is two millimeters larger than the last reamer size used. Inserting this size cup into the undersized reamed acetabulum to accommodate this system is sometimes difficult, particularly with resistance in the dome area of the cup, which is also larger than the last reamer size used.
In order to avoid cementing or press fitting an acetabular cup, and the associated problems described above, it is known for an acetabular cup prosthesis to include one or more flanges that are to be matingly attached to respective bones of the patient's pelvis that surround the acetabulum, i.e., the ischium, pubis and ilium. The mating surfaces of the flanges have undulations that follow the curvature of the bones to which the flanges are attached. Ideally, the flanges should have uniform thicknesses in order to maximize the flanges' strength as well as the volume of space adjacent to the flanges that is available for muscle and tissue. However, as described in more detail, below, a practical method of providing the flanges with uniform thicknesses has not thus far been achieved.
One techniques for designing the flanges involves obtaining three dimensional data defining the patient's natural hip joint via computed tomography (CT), commonly known as a CAT scan. From the CT data, a stereolithography model is made of at least a portion of the patient's natural hip joint. Clay is then pressed against the model of the hip joint in order to form a clay model of the prosthesis that is to be implanted into the patient's natural hip joint. A laser scan of the clay model is used to create an implantable embodiment of the clay model prosthesis. This prosthesis is then implanted into the patient's hip joint.
A problem with the above-described technique is that the clay models of the prosthesis can have imperfections, such as an insufficiently uniform thickness, particularly due to the model-maker's inability to see or otherwise determine the thickness of the clay at all points. When the steel prosthesis is fabricated as a replication of the clay model, its flanges also may not have sufficiently uniform thicknesses, which results in either thin areas subject to cracking or overly thick areas that deprive the muscle and tissue of needed space.
It is also known with such hip replacements to perform a CT scan on the other hip, i.e, the “good” hip, and make a stereolithographic model of the hip therefrom. From the model, the angular orientation of the acetabular cup can be measured and used to set the angular orientation of the prosthetic acetabular cup to be implanted in the “bad” hip. A first problem with this technique is the difficulty in accurately measuring the angular orientation of the acetabulum in the good hip, and replicating the angular orientation in the prosthesis to be inserted into the bad hip. A second problem is the substantial additional cost involved with making a model of the good hip.
It is also known to use computer aided design (CAD) software to design other types of prostheses based upon imported data obtained from a CT scan of a patient's body. For example, U.S. Pat. Nos. 4,436,684, 5,741,215, 5,798,924 and 6,254,639, are directed to designing an entire prosthesis based upon the CT data. However, using these prior approaches to creating a unique prosthesis design for each patient can result in unforeseeable problems and takes away the familiarity that the surgeon will likely have with standardized prosthesis designs. Thus, prosthesis designs that are entirely customized are considered sub-optimal solutions.
The use of CT data in the above patents is generally confined to the design of customized prostheses that replace a portion of bone that has been lost due to trauma or deterioration. These prior approaches are not directed to using CT data to design a prosthesis that includes an attachment part for attaching or for otherwise associating a standardized functional part with a traumatized or deteriorated bone.
Consequently, there is a need for a method of designing and manufacturing a customized prosthesis that addresses these and other drawbacks. With respect to the hip joint, there is a particular need for an acetabular prosthesis and associated method of making an acetabular prosthesis that overcomes one or more of the above-mentioned problems. More particularly, what is needed is an acetabular prosthesis and associated method of fabrication that enables the acetabular cup to be secured to the acetabulum of the innominate bone without the cup being press fit into a reamed hole in the acetabulum, and without the use of bone cement.
There is also a need for a prosthesis and associated method of fabrication that provides a flange of the prosthesis with a uniform thickness and a bone-facing surface that is complementary to a portion of the surface of the patient's bone. Again, with respect to the hip, a need remains for an acetabular prosthesis and associated method of fabrication that provides an acetabular cup with an angular orientation relative to the flanges and/or the patient's bone structure that is accurately based upon a corresponding angular orientation in the contralaterally corresponding hip.